Add io.include() function to Nasal base library

io.inlude() loads and executes a Nasal file in place, effectively embedding
the script in the calling namespace. The function adds a symbol mark in the
namespace to avoid duplicate loading.

Additionally, in this path:

 + io.basename() & io.dirname(): Convenience functions for managing path
   strings. Designed after their unix counterparts.

 + string.normpath(): Improved support for relative paths. It can now handle
   paths starting with double dots, like ../../Directory